Outer rotor type motor for drum type washing machine and method for fabricating the same

ABSTRACT

Direct coupling type motor for drum type washing machine having a stator which can reduce material required for fabrication, and a weight after fabrication, simplify a fabrication process, and can be mounted on a fixing side, such as a tub or a bearing housing securely, and a rotor which can enhance rigidity an outer rotor and protect a surface of the outer rotor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/008,407, filed Dec. 10, 2004, now U.S. Pat. No. 7,294,942, whichclaims priority to Korean Patent Application Nos. 2003-89721,2003-89722, 2003-89723, 2003-89724 and 2003-89726 all filed Dec. 10,2003, and all of which are incorporated by reference as if fully setforth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to drum type washing machines, and moreparticularly, to an outer rotor type BLDC motor applicable to a directcoupling type drum type washing machine, and a method for fabricatingthe same.

2. Discussion of the Related Art

In general, a drum type washing machine washes laundry by using afriction force between a drum rotated by a driving power of a motor andlaundry in a state detergent, washing water, and the laundry areintroduced into the drum, shows almost no damage to, and entangling ofthe laundry, and has pounding, and rubbing washing effects.

In the related art drum type washing machines, there are an indirectcoupling type in which the driving power is transmitted from the motorto the drum through a belt wound on a motor pulley and a drum pulleyindirectly, and a direct coupling type in which a rotor of a BLDC motoris coupled to the drum directly, to transmit the driving power from themotor to the drum, directly.

The type in which the driving power of the motor is transmitted to thedrum, not directly, but indirectly through the motor pulley and the drumpulley, has much energy loss in the course of power transmission, andcauses much noise in the course of power transmission.

According to this, for solving the problems of the indirect couplingtype drum type washing machines, it is the present trend that use of thedirect coupling type drum type washing machines with the BLDC motor isincreasing, as example of which there are Korean Laid Open Patent Nos.2001-37517, and 2001-37518.

A related art direct coupling type drum type drum type washing machineand a structure of a motor thereof will be described with reference toFIGS. 1 to 6, briefly. FIG. 1 illustrates a longitudinal section of arelated art drum type washing machine.

Referring to FIG. 1, the related art drum type washing machine isprovided with a tub 2 mounted on an inside of a cabinet 1, and a drum 3rotatably mounted on a central portion of an inside of the tub 2. Thereis a motor in rear of the tub 2, wherein a stator 6 is secured to a rearwall of the tub, and a rotor 5 surrounds the stator 6, and is connectedto the drum 3 with a shaft passed through the tub.

In the meantime, there are a door 21 mounted on a front of the cabinet1, and a gasket 22 between the door 21 and the tub 2.

There are hanging springs 23 between an inside surface of an upperportion of the cabinet 1, and an upper portion of an outsidecircumferential surface of the tub 2, and a friction damper 24 betweenthe inside surface of a lower portion of the cabinet 1, and a lowerportion of the outside circumferential surface of the tub 2.

FIG. 2 illustrates a perspective exterior view of the stator in FIG. 1,and FIG. 3 illustrates a perspective view of a divisional type core DCapplied to the stator in FIG. 2.

In a related art method for fabricating the core, a sheet of metal plateis pressed to form a unit core having Ts 151, a base portion 150, andprojected portions 500 opposite to the Ts 151 each for forming fasteninghole 500 a therein, the unit cores are stacked to form a unit coreassembly, and the unit core assemblies are connected to each other in acircumferential direction, to complete fabrication of the stator core,called the divisional core DC.

The projected portion 500 provides the fastening hole 620 a forfastening the stator 6 to the rear wall of the tub, and serves to endurea fastening force of a bolt.

However, the method for fabricating the stator 6 by means of thedivisional cores DC has, not only a complicate fabrication process, butalso loss of much material.

Therefore, even if a helical type core HC is favorable, in which a sheetof steel plate having the Ts 151 and the base portions 150 is stackedturning in a helix for reducing the material loss, and making thefabrication process simple, since it is required to bend the sheet ofmetal punched out in a shape of a stripe into the helix, the helicalcore has a drawback in that the projected portion for fastening thestator to the tub can not be formed on an inner side of the core.

This is because, if the projected portion 500 is formed on the innerside of the core in fabrication of the helical core HC, a large width ofthe core at a portion having the projected portion formed thereonimpedes bending of the core.

Therefore, currently, for employing the helical type core HC, a statorstructure is required, in which a function the same with the projectedportion of the divisional core DC is made to be carried out, not by thecore itself, but by other portion.

For reference, a reason why it is important to secure an adequaterigidity of the projected portion having the fastening hole formed infor fastening the stator to the tub is as follows.

The washing machine in which the drum is directly rotated by the BLDCmotor has the stator mounted on a rear portion of the tub, directly. Ina case of the motor for a large capacity drum type washing machine withmore than 1.5 kg of stator net weight, and a spinning speed in a rangeof 600˜2000 RPM, it is liable that a fastened portion of the stator 6 isbroken due to the stator weight, and vibration, shaking, and deformationof the rotor 5 in the high speed rotation.

Particularly, in a case of the drum type washing machine, in which theBLDC motor is used, and the stator 6 is secured to the tub rear wall,where an axis direction of the stator 6 is substantially parallel toground, the vibration generated during operation of the washing machinecauses intensive damage to the fastening portion of the stator 6 to thetub rear wall.

Thus, an adequate rigidity of the projected portion having the fasteninghole formed therein is very important in fastening the stator 6 to thetub.

A related art outer rotor will be described with reference to FIGS. 5and 6.

Referring to FIGS. 5 and 6, because the outer rotor R pressed of a steelplate (hereafter called as “steel plate rotor”) has a stepped portionalong a circumferential direction formed on a sidewall 120 extended froma circumference of a bottom 110 of the rotor frame 100 perpendicular toa bottom 100 surface, to support magnets M when the magnets M areattached to an inside surface of the sidewall 120 of the rotor frame100, fabrication of the outer rotor is easy.

Moreover, there are a plurality of radial cooling fins 130 around acenter portion of the bottom 110 of the rotor frame 100 of the steelplate rotor R to blow air toward the stator (not shown) to cool downheat generated at the stator.

The cooling fins 130 are lanced toward an opened side of the rotor, andopenings formed by the lancing serve as vent holes.

There are embossed portions 150 between adjacent cooling fins 130 on thebottom 110 of the rotor frame 100 for reinforcing strength of the rotor,and there are drain holes 160 in the embossed portions 150 for drainingwater.

However, the related art steel plate rotor R has the following problems.

Despite of anti-rusting heat treatment of a surface of the related artsteel plate rotor R, rust occurs on a portion of the surface scratchedduring transportation or other situations.

The surface of the rotor also corrodes due to chemical reaction ofdetergent stuck to the surface of the rotor as the drum type washingmachine is used for a long time.

In the meantime, during the spinning for water extraction, the sidewall120 of the related art steel plate rotor 100 throbs intensely due toelectro-magnetic interaction (attraction and repulsion) with the stator,leading to increase noise.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an outer rotor typeBLDC motor for a drum type washing machine, and a method for fabricatingthe same that substantially obviates one or more problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide an outer rotor typeBLDC motor for a drum type washing machine, which has a stator that canreduce material and weight required for fabrication, has a simplefabrication process, and can be mounted on a fixed side, such as a tubor a bearing housing securely.

Another object of the present invention is to provide a new statorstructure which can be mounted on a fixed side, such as a tub or abearing housing securely while reducing material for fabrication asdescribed before so as to be suitable for a BLDC motor of a drum typewashing machine, which has a weight over 1.5 kg only of the stator, anda rotation speed varying 0˜2,000 RPM or over.

Another object of the present invention is to provide an outer rotortype BLDC motor for a drum type washing machine, which can preventscratch on a surface of the steel plate outer rotor from occurringeffectively, and cut off reaction of the surface with detergenteffectively, for extending a lifetime of the motor, and improvingreliability.

A further object of the present invention is to provide an outer rotortype BLDC motor for a drum type washing machine, which can enhance arigidity of the outer rotor, to solve the problem of throbbing andconsequential noise.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, anouter rotor type motor for a drum type washing machine includes anannular stator core having multiple layers formed by winding a baseportion in a shape of stripe with Ts projected from the base portion ina helix starting from a bottom layer to a top layer, an insulator insertmolded to cover an outside surface of the stator core for insulating thestator core electrically, and fastening portions formed as one body withthe insulator each projected from an inside circumferential surface ofthe stator core toward a center of the stator, for fixedly securing thestator to the tub, wherein the base portion of the stator core has anarc shaped slots each for reducing stress in winding the core as well asserving as a portion of a fastening hole, and each of the fasteningportion of the insulator formed to cover the slots has a fastening holeformed therein.

In another aspect of the present invention, an outer rotor type motorfor a drum type washing machine includes an annular stator core havingmultiple layers formed by winding a steel plate with a stripe shape ofbase portion and Ts projected from the base portion in a helix startingfrom a bottom layer to a top layer such that a layer overlaps with anext layer, an upper insulator of an electric insulating materialcovered on an upper side of the stator core in a shape complementary toa shape of the stator core, and a lower insulator of an electricinsulating material covered on a lower side of the stator core at thetime of assembly with the upper insulator in a shape complementary to ashape of the stator core, wherein each of the upper insulator and thelower insulator includes three or more than three fastening portionsformed as one body therewith projected from an inside of the stator coretoward a center of the stator for fastening the stator to a fixing sideof the tub, and the fastening portion has a fastening hole for securingthe stator to a tub wall with fastening members.

In another aspect of the present invention, an outer rotor type motorfor a drum type washing machine includes an annular stator core havingmultiple layers formed by winding a steel plate with a stripe shape ofbase portion and Ts projected from the base portion in a helix startingfrom a bottom layer to a top layer such that a layer overlaps with anext layer, an upper insulator of an electric insulating materialcovered on an upper side of the stator core in a shape complementary toa shape of the stator core, and a lower insulator of an electricinsulating material covered on a lower side of the stator core at thetime of assembly with the upper insulator in a shape complementary to ashape of the stator core, wherein each of the upper insulator and thelower insulator includes three or more than three fastening portionsformed as one body therewith projected from an inside of the stator coretoward a center of the stator for fastening the stator to a fixing sideof the tub, the fastening portion has a boss of a fastening hole forsecuring the stator to a tub wall, and there is a cylindrical metalplaced around the boss.

In another aspect of the present invention, an outer rotor type motorfor a drum type washing machine includes an annular stator core havingmultiple layers formed by winding a steel plate with a stripe shape ofbase portion and Ts projected from the base portion in a helix startingfrom a bottom layer to a top layer such that a layer overlaps with anext layer, an upper insulator of an electric insulating materialcovered on an upper side of the stator core in a shape complementary toa shape of the stator core, and a lower insulator of an electricinsulating material covered on a lower side of the stator core at thetime of assembly with the upper insulator in a shape complementary to ashape of the stator core, annular supporters respectively inserted inthe upper insulator and the lower insulator; wherein each of the upperinsulator and the lower insulator, and the supporters inserted thereinincludes fastening portions projected from an inside of the stator coretoward a center of the stator for fastening the stator to a fixing sideof the tub, and the fastening portion has a fastening hole for securingthe stator to a tub wall with fastening members.

In further aspect of the present invention, an outer rotor type motorfor a drum type washing machine includes a stator core stacked bywinding in a helix, an insulator of an insulating material having anupper side piece, and a lower side piece to cover the core when theupper side piece, and the lower side piece are joined together, anannular supporter of metal inserted in at least one of the upperinsulator and the lower insulator, and fastening portions formed as onebody with the upper, and lower insulators, and/or the supporter insertedtherein, each of the fastening portions being projected from an insideof the stator core toward a center thereof for securing the stator to afixing side of the tub.

In still further aspect of the present invention, an outer rotor typemotor for a drum type washing machine includes an outer rotor of steelplate having a bottom, a sidewall extended from the bottom inperpendicular thereto substantially, and magnets on an inside of thesidewall, wherein the rotor frame and a back yoke are formed as one unitso that the rotor frame also serves as the back yoke, and the rotorframe has one coat of synthetic resin applied to at least an outsidesurface of the rotor frame.

In the meantime, in still further aspect of the present invention, anouter rotor type motor for a drum type washing machine includes an outerrotor of steel plate having a bottom, a sidewall extended from thebottom in perpendicular thereto substantially, and magnets on an insideof the sidewall, wherein an annular back yoke is mounted on an insidesurface of the sidewall of the outer rotor, and the magnets are attachedto an inside surface of the back yoke.

In still yet further aspect of the present invention, a method forfabricating an outer rotor type motor for a drum type washing machineincludes the steps of forming an annular stator core having multiplelayers formed by winding a steel plate with a stripe shape of baseportion and Ts projected from the base portion in a helix starting froma bottom layer to a top layer such that a layer overlaps with a nextlayer, insert molding the stator core such that an insulating materialcovers the stator core, and an inside of a slot forms a portion of afastening hole, and winding coils on Ts of the stator coil.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings;

FIG. 1 illustrates a longitudinal section of a related art directcoupling type drum type washing machine, schematically;

FIG. 2 illustrates a perspective view of an exterior of the stator inFIG. 1;

FIG. 3 illustrates a perspective view of the divisional core in FIG. 2;

FIG. 4 illustrates a reference drawing showing a fabrication process ofthe divisional core in FIG. 3;

FIG. 5 illustrates a perspective view of the outer rotor in FIG. 1, witha partial cut away view;

FIG. 6 illustrates a section of FIG. 5;

FIG. 7 illustrates a perspective view of an exterior of the stator inaccordance with a first preferred embodiment of the present invention;

FIG. 8 illustrates a perspective view of the core in the stator in FIG.7;

FIG. 9 illustrates a plan view of key portions of FIG. 7;

FIG. 10 illustrates a cross sectional view of FIG. 9, showing an insideof a fastening portion for reference;

FIGS. 11 and 12 illustrate cross sectional views equivalent to FIG. 10,showing key portions of slots in accordance with another preferredembodiments of the present invention;

FIG. 13 illustrates an exterior perspective view of a stator inaccordance with a second preferred embodiment of the present invention;

FIG. 14 illustrates a disassembled perspective view of FIG. 13;

FIG. 15 illustrates a bottom perspective view of a portion of the upperinsulator in FIG. 14;

FIG. 16 illustrates a section showing a fastening portion across a lineI-I in FIG. 13;

FIGS. 17 and 18 are reference drawings, illustrating a versatility ofapplication of the insulators;

FIG. 19 illustrates a section of a fastening portion corresponding toFIG. 16 in accordance with another preferred embodiment;

FIG. 20 illustrates a perspective view of an exterior of a stator inaccordance with a third preferred embodiment of the present invention;

FIG. 21 illustrates a disassembled perspective view of FIG. 20;

FIG. 22 illustrates a plan view of an upper insulator;

FIG. 23 illustrates a plan view of a lower insulator;

FIG. 24 illustrates a perspective view of a bottom of a portion of theupper insulator in FIG. 21;

FIG. 25 illustrates a section of a fastening portion across a line II-IIin FIG. 21;

FIG. 26 illustrates a perspective view of a supporter of a metal ringshape applicable to the stator in FIG. 20;

FIG. 27 illustrates a perspective view of an outer rotor in accordancewith a fourth preferred embodiment of the present invention, with apartial cut away view;

FIG. 28 illustrates a section of FIG. 27;

FIG. 29 illustrates a perspective view of an outer rotor in accordancewith a fifth preferred embodiment of the present invention, with apartial cut away view; and

FIG. 30 illustrates a section of FIG. 29.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings FIGS. 7 to 34. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

A first embodiment of the present invention will be described.

FIG. 7 illustrates a perspective view of an exterior of the stator inaccordance with a first preferred embodiment of the present invention,FIG. 8 illustrates a perspective view of the core in the stator in FIG.7, FIG. 9 illustrates a plan view of key portions of FIG. 7, and FIG. 10illustrates a cross sectional view of FIG. 9, showing an inside of afastening portion for reference.

The stator 6 of a direct coupling type motor in accordance with a firstpreferred embodiment of the present invention includes an annular statorcore 15 having multiple layers formed by winding a steel plate with Ts151 and base portion 150 in a helix starting from a bottom layer to atop layer such that a layer overlaps with the next layer, an insulator144 insert molded to cover an outside surface of the stator core forinsulating the stator core electrically, and three or more fasteningportions 143 formed as one body with the insulator 144 each projectedfrom an inside circumferential surface of the stator core 15 toward acenter of the stator 6, for fixedly securing the stator 6 to the tub 2,wherein the base portion 150 of the stator core 15 has arc shaped slots152 each for reducing stress in winding the core as well as serving as aportion of a fastening hole 143 a, and each of the fastening portion 143of the insulator 144 formed to cover the slots 152 has a fastening hole143 a formed therein.

A process for fabricating the stator in accordance with a preferredembodiment of the present invention will be described.

A Core having the stripe shaped base portion 150 with the arc shapedslots 152, and Ts 151 projected from the base portion 150 is punchedfrom an electric sheet, a base metal. In this instance, of the core isstraight.

Then, the punched core is wound in a helix from a bottom layer to a toplayer such that a layer overlaps with the next layer, to form a multiplelayered annular stator core 15.

In view of a geometric characteristic, the stator core 15 is also calledas a helical core HC, and the multiple layers thereof are fastened withrivets 153 passed through pass through holes in the base portion 150.

The arc shaped slots 152 reduce stress during winding the core, enablingto wind the core with a smaller force.

Then, an insert molding is performed to cover the stator core 15 with aninsulating material, such as plastic, or the like.

The insulating material covers the stator core 15 such that an inside ofthe slot 152 forms a portion of the fastening hole 143 a.

That is, in an inside direction of the slot 152, there is the fasteningportion 143 of the insulator 144, and the fastening portion has a boltfastening hole 143 a.

After finishing the insert molding, a coil is wound around each of theTs 151 of the core 15.

The foregoing stator core in accordance with a first preferredembodiment of the present invention works as follows.

Different from the divisional core, the application of so called helicalcore HC, formed by stacking a steel plate having Ts 151 and a baseportion 150 while winding in a helix, to the embodiment as the statorcore 15 permits the embodiment to omit steps of aligning, and weldingthe core segments, to simplify a fabrication process.

Moreover, different from the divisional core, since the stator core 15has no projected portion, the stator core 15 permits to reduce waste ofmaterial.

That is, the method for fabricating a stator in accordance with theembodiment not only has a simple fabrication process, but also permitsto reduce waste of material.

Furthermore, by improving structures of the stator core 15 and theinsulator 144, the stator 6 has an adequate rigidity against boltfastening force even is the stator 6 has no projected portion on thecore itself like the divisional core DC.

That is, the arc shaped slot 152 is formed in an inside surface of thehelical type stator core, and the insulator 144 is formed on an exteriorof the core by insert molding, wherein the insulator 144 on the exteriorof the core forms a circular hole together with the arc shaped slot onthe inside of the core.

That is, since the stator core 15 has a shape of a helical core HC withan inside of the slot 152 being a portion of the bolt fastening hole 143a, at the time of bolt fastening for securing the stator 6 to the tub 2,a portion of bolt head of the bolt presses down an upper surface of theinsert molded core, and rest of the bolt head presses down an uppersurface of the fastening portion 143 of the insulator 144.

According to this, the stator 6 can effectively prevent a fasteningportion of the stator 6 suffering from breakage caused by vibration atthe spinning, and shaking and deformation of the rotor 5 even at a largecapacity drum type washing machine having a weight over 1.5 kg only ofthe stator, and a spinning speed ranging 600˜2,000 RPM.

As a positioning projection 143 b in the vicinity of the fastening hole143 a of the fastening portion 143 fits in a positioning hole (notshown) in the tub 2, fastening of the stator 6 is easy.

Thus, the positioning projection enables an easy assembly of the stator6 with the tub 2, and a serviceman to make an easy repair at the time ofafter service.

Of course, the positioning projection may be formed on the tub 2, andthe positioning hole may be formed in the fastening portion 143.

FIGS. 11 and 12 illustrate cross sectional views equivalent to FIG. 10,showing key portions of slots in accordance with another preferredembodiments of the present invention, wherein it can be noted that theslot 152 in the base portion 150 of the stator core 15 may be greaterthan a semicircle as shown in FIG. 11 or smaller than semicircle asshown in FIG. 12.

In other words, a center of a radius of the slot 152 may be positionedon an inside, or outside of the slot.

That is, more than one half of an area of the fastening hole 143 a maybe positioned on an inside of the slot as shown in FIG. 11, or on anoutside of the slot as shown in FIG. 12.

In a case of FIG. 11, a contact area between the bolt head and thestator core 15 is large, to increase a supporting force against the boltfastening force.

Moreover, the case permits an easy bending of the core at the time ofwinding the stator core in a helix.

A case of FIG. 12 is applicable to a general small sized motor becausethe contact area between the bolt head and the stator core 15 is small,to reduce the supporting force against the bolt fastening force.

Along with this, in the vicinity of the fastening hole 143 a of thefastening portion, there is a positioning projection 143 b incomplementary to a positioning hole (not shown) in the tub 2.

A second preferred embodiment of the present invention will be describedwith reference to FIGS. 13˜19.

FIG. 13 illustrates an exterior perspective view of a stator inaccordance with a second preferred embodiment of the present invention,FIG. 14 illustrates a disassembled perspective view of FIG. 13, FIG. 15illustrates a bottom perspective view of a portion of the upperinsulator in FIG. 14, and FIG. 16 illustrates a section showing afastening portion across a line I-I in FIG. 13.

The stator 6 of an outer rotor type motor in accordance with a secondpreferred embodiment of the present invention includes an annular statorcore 15 having multiple layers formed by winding a steel plate with astripe shape of base portion 150 and Ts 151 projected from the baseportion 150 in a helix starting from a bottom layer to a top layer suchthat a layer overlaps with a next layer, an upper insulator 60 a of anelectric insulating material covered on an upper side of the stator core15 in a shape complementary to a shape of the stator core 15, and alower insulator 60 b of an electric insulating material covered on thelower side of the stator core 15 at the time of assembly with the upperinsulator 60 a in a shape complementary to a shape of the stator core15, wherein each of the upper insulator 60 a and the lower insulator 60b includes three or more than three fastening portions 600 formed as onebody therewith projected from an inside of the stator core 15 toward acenter of the stator 6 for fastening the stator 6 to a fixing side ofthe tub.

The fastening portion 600 (see FIG. 14) has a fastening hole 620 a (seeFIG. 15) for securing the stator 6 to a fixing side, such as the tub,with a fastening member. The fastening hole 620 a is constructed of aboss 620 projected to a back side of the fastening portion 600.

Each of the upper insulator 60 a and the lower insulator 60 b has asupporting rib 650 on an inside along a circumferential directionthereof in contact with the stator core 15 for supporting an insidesurface of the core.

The fastening portion 600 of each of the upper insulator 60 a and thelower insulator 60 b has at least one reinforcing rib 660 connectedbetween the boss 620 of the fastening hole 520 a and the supporting rib650 for spreading fastening force concentrated on the boss 620 andreinforcing a strength of the fastening portion 600.

In the meantime, the fastening portion 600 of each of the upperinsulator 60 a and the lower insulator 60 b has a reinforcing rib 670 atan inside circumference thereof, and at least one connection rib 680connected between the reinforcing rib 670 and the supporting rib 650which supports an inside surface of the core in a radial direction, forproviding a supporting force.

The reinforcing rib 670, together with the reinforcing rib 660, connectsbetween the bosses 620 having fastening holes of the upper, and lowerinsulators, to spread the fastening force concentrated on the boss, andreinforce a strength of the fastening portion 600.

In the meantime, each of the upper insulator 60 a and the lowerinsulator 60 b has tips 610 a and 610 b on opposite sidewalls of each ofthe Ts 610 thereof having shapes in complementary to each other forfitting in at the time of assembly to form a flush surface.

Each of the tips 610 a and 610 b on each of the Ts 610 has a “

” shape if the other side has a “

” shape.

At the opposite end surfaces substantially perpendicular to the oppositesidewall surfaces of the teeth 610 of the upper insulator 60 a and thelower insulator 60 b, there are also the tips 610 a and 610 b havingshape in complementary to each other.

The teeth 610 of each of the upper insulator 60 a and the lowerinsulator 60 b has a seating surface 611 a at an end for seating a coreshoe 151 a of the stator core 15.

Along with this, in the vicinity of the fastening hole 620 a of thefastening portion 600 of the upper insulator 60 a, there is apositioning projection 630 having a shape in complementary to apositioning hole or a slot (not shown) in the fixing side of the tub.

There is a cylindrical metal 800 in the fastening hole 620 a, a springpin having an elasticity owing to an incised portion, or a hollow pinenabling press fit in the fastening hole 620 a, serves as a bushing.

In the meantime, the fastening hole 620 a of each of the upper insulator60 a and the lower insulator 60 b has a land around the fastening hole620 a for preventing the bolt head from being brought into directcontact with the cylindrical metal 800 at the time of bolt fastening.

The base portion 150 of the stator core 15 has slots 152 for reducingstress at the time of winding in formation of the core to make thewinding easy, and the multiple layers thereof are fastened with rivets153 passed through pass through holes in the base portion 150.

In FIG. 14, an unexplained reference numeral 8 denotes a hole sensorassembly for controlling the motor, and 9 denotes a tap housing assemblyfor tapping power to supply the power to a stator side.

The work of the second embodiment stator will be described.

Different form the divisional core DC, the application of so calledhelical core HC, formed by stacking a steel plate having Ts 151 and abase portion 150 while winding in a helix, to the embodiment as thestator core 15 also permits the embodiment to omit steps of aligning,and welding the core segments, to simplify a fabrication process.

Moreover, different from the divisional core, since the stator core 15has no projected portion, the stator core 15 permits to reduce waste ofmaterial.

That is, a method for fabricating a stator of the embodiment not onlysimplifies a fabrication process, but also reduces waste of material.

Moreover, even if the stator 6 of the embodiment improves structures ofthe upper, and the lower insulator 60 a, and 60 b such that no projectedportions are formed at the core itself for supporting the fasteningforce at the time of fastening the stator 6 to the tub side, the stator6 still has a rigidity enough to support the bolt fastening force.

That is, by providing structures that work the same with the projectedportion of the divisional core to the fasting portions 600 of the upper,and lower insulators 60 a, and 60 b, a stator 6 can be provided, towhich the helical core HC is applicable.

Moreover, spaces 640 between the ribs 650, 660, 670, and 680 at a backside of the fastening portion 600 dampen and attenuate vibrationoccurred during driving the motor, to improve mechanical reliability ofthe stator 6, and contributes to save material of the insulators.

The supporting ribs 650 of the upper insulator 60 a and the lowerinsulator 60 b formed at an inside in contact with the stator core 15along the circumferential direction support an inside surface of thestator core 15.

The reinforcing rib 660 connected between the boss 620 of the fasteninghole 620 a and the supporting rib 650 at each of the fastening portion600 of the upper insulator 60 a and the lower insulator 60 b spreads thefastening force concentrated on the boss 620, and reinforces a strengthof the fastening portion 600.

According to this, the stator 6 can effectively prevent a fasteningportion of the stator 6 suffering from breakage caused by vibration atthe spinning, and shaking and deformation of the rotor 5 even at a largecapacity drum type washing machine having a weight over 1.5 kg only ofthe stator, and a spinning speed ranging 600˜2,000 RPM.

As a positioning projection 630, or 143 b in the vicinity of thefastening hole 620 a of the fastening portion 600 fits in a positioninghole (not shown) in the tub 2, fastening of the stator 6 is easy.

Thus, the positioning projection enables an easy assembly of the stator6 with the tub 2, and a serviceman to make an easy repair at the time ofafter service.

Of course, the positioning projection 630 may be formed on the tub 2,and the positioning hole may be formed in the fastening portion 600.

In the meantime, FIGS. 17 and 18 are reference drawings, illustrating aversatility of application of the insulators, wherein it can be notedthat the upper, and lower insulators 60 a and 60 b are applicable evenif a total height of the helical core varies within a certain range.

That is, FIG. 17 illustrates a case when a total height h1 of the coreis a height which permits tips 610 a and 610 b of the upper, and lowerinsulators 60 a and 60 b fit exactly, and FIG. 18 illustrates a casewhen a total height of the core is a height greater than a case of FIG.17 such that the tips 610 a (see FIGS. 14 and 15) and 610 b (see FIG.15) of the upper, and lower insulators 60 a and 60 b unable to fitexactly, but spaced a certain distance.

Even if the total height h2 of the core is greater than a height thatpermits the tips 610 a and 610 b of the upper, and lower insulators 60 aand 60 b fit exactly, such that there is a space between the tips 610 aand the 610 b, because the insulation against the core Ts is stillachievable, the upper, and lower insulators 60 a and 60 b are applicableto the case of FIG. 18, too.

Thus, since the separate type upper, and lower insulators of theembodiment are applicable to the core regardless of the total height ofthe core within a certain range of the total height, the separate typeupper, and lower insulators of the embodiment can improve workability onan assembly line.

FIG. 19 illustrates a section of a fastening portion corresponding toFIG. 16 in accordance with another preferred embodiment, wherein acylindrical metal 800 is mounted on an outside of the boss 620 of thefastening hole 620 a in the fastening portion 600 of each of the upper,and lower insulators 60 a, and 60 b.

That is, in this embodiment, a boss 620 of the fastening hole 620 awhich is formed to secure the stator 6 to a fixing side, such as thetub, is formed on a back side of the upper and lower insulators 60 a,and 60 b, and a cylindrical metal 800 is placed around the boss. Thisplacement of the cylindrical metal on the outside of the boss is madepossible owing to the separable upper, and lower insulators.

A third embodiment stator core will be described with reference to FIGS.20˜26.

FIG. 20 illustrates a perspective view of an exterior of a stator inaccordance with a third preferred embodiment of the present invention,FIG. 21 illustrates a disassembled perspective view of FIG. 20, FIG. 22illustrates a plan view of an upper insulator, and FIG. 23 illustrates aplan view of a lower insulator.

FIG. 24 illustrates a perspective view of a bottom of a portion of theupper insulator in FIG. 21, FIG. 25 illustrates a section of a fasteningportion across a line II-II in FIG. 21, and FIG. 26 illustrates aperspective view of a supporter of a metal ring shape applicable to thestator in FIG. 20.

Referring to FIGS. 20˜26, the stator 6 of an outer rotor type motor inaccordance with a third preferred embodiment of the present inventionincludes an annular stator core 15 having multiple layers formed bywinding a steel plate with a stripe shape of base portion 150 and Ts 151projected from the base portion 150 in a helix starting from a bottomlayer to a top layer such that a layer overlaps with a next layer, anupper insulator 60 a of an electric insulating material covered on anupper side of the stator core 15 in a shape complementary to a shape ofthe stator core 15, a lower insulator 60 b of an electric insulatingmaterial covered on the lower side of the stator core 15 at the time ofassembly with the upper insulator 60 a in a shape complementary to ashape of the stator core 15, and annular supporters 900 of metalinserted to the upper insulator and the lower insulator, respectively.

Each of the upper insulator, the lower insulator, and the supporters 900inserted in the upper insulator and the lower insulator respectively hasthree or more than three fastening portions 600 projected from an insideof the stator core 15 toward the center of the stator 6 for securing thestator 6 to a fixing side of the tub.

The fastening portion 600 has a fastening hole 620 a for securing thestator 6 to a fixing side, such as the tub, with fastening members. Thefastening hole 620 a is constructed of a boss 620 projected on a backside of the fastening portion 600.

Each of the upper insulator 60 a and the lower insulator 60 b has asupporting rib 650 on an inside along a circumferential directionthereof in contact with the stator core 15 for supporting an insidesurface of the core.

The fastening portion 600 of each of the upper insulator 60 a and thelower insulator 60 b has at least one reinforcing rib 660 connectedbetween the boss 620 of the fastening hole 620 a and the supporting rib650 for spreading fastening force concentrated on the boss 620 andreinforcing a strength of the fastening portion 600.

In the meantime, it is preferable that the fastening portion 600 of eachof the upper insulator 60 a and the lower insulator 60 b has areinforcing rib 670 at an inside circumference thereof in the back side,and at least one connection rib 680 connected between the reinforcingrib 670 and the supporting rib 650 which supports an inside surface ofthe core in a radial direction, for providing a supporting force.

In the meantime, each of the upper insulator 60 a and the lowerinsulator 60 b has tips 610 a and 610 b on opposite sidewalls of each ofthe Ts 610 thereof having shapes in complementary to each other forfitting in at the time of assembly.

Each of the tips 610 a and 610 b on each of the Ts 610 has a “

” shape if the other side has a “

” shape.

At the opposite end surfaces substantially perpendicular to the oppositesidewall surfaces of the teeth 610 of the upper insulator 60 a and thelower insulator 60 b, there are also the tips 610 a and 610 b havingshape in complementary to each other.

The teeth 610 of each of the upper insulator 60 a and the lowerinsulator 60 b has a seating surface 611 a at an end for seating a coreshoe 151 a of the stator core 15.

Along with this, in the vicinity of the fastening hole 620 a of thefastening portion 600 of the upper insulator 60 a, there is apositioning projection 630 having a shape in complementary to apositioning hole or a slot (not shown) in the fixing side of the tub.

The metal supporters 900 inserted in the upper insulator 60 a and thelower insulator 60 b serve as portions to support the fastening forcewhen the fastening force is applied for securing the stator 6 to a tubwall.

In the meantime, the fastening hole 620 a of each of the upper insulator60 a and the lower insulator 60 b has a land around the fastening hole620 a for landing of the bolt head at the time of bolt fastening.

There may be a metal tube or a spring pin on an inside of the fasteninghole 620, for reinforcing strength of the fastening portion.

The base portion 150 of the stator core 15 has slots 152 for reducingstress at the time of winding in formation of the core to make thewinding easy, and the multiple layers thereof are fastened with rivets153 passed through pass through holes in the base portion 150.

In FIG. 21, unexplained reference numeral 8 denotes a hole sensorassembly for controlling the motor, and 9 denotes a tap housing assemblyfor tapping power to supply the power to a stator side.

The work of the third embodiment stator will be described.

Different form the divisional core DC, the application of so calledhelical core HC, formed by stacking a steel plate having Ts 151 and abase portion 150 while winding in a helix, to the embodiment as thestator core 15 permits the embodiment to omit steps of aligning, andwelding the core segments, to simplify a fabrication process.

Moreover, different from the divisional core, since the stator core 15has no projected portion, the stator core 15 can reduce waste ofmaterial.

That is, a method for fabricating a stator of the embodiment not onlysimplifies a fabrication process, but also reduces waste of material.

Moreover, even if the stator 6 of the embodiment improves structures ofthe upper, and the lower insulator 60 a, and 60 b such that no projectedportions are formed at the core itself for supporting the fasteningforce at the time of fastening the stator 6 to the tub side, the stator6 still has a rigidity enough to support the bolt fastening force.

That is, by providing structures that work the same with the projectedportion of the divisional core to the fasting portions 600 of the upper,and lower insulators 60 a, and 60 b, and the fastening portions 600 ofthe supporter 900 inserted in the upper, and lower insulators 60 a and60 b, a stator 6 can be provided, to which the helical core HC isapplicable.

Especially, different from the second embodiment, because the supporters900 are inserted in the upper, and lower insulators 60 a, and 60 badditionally, which are formed of metal, such as steel plate, the statorcore of the third embodiment have adequate rigidity against the boltfastening force.

Moreover, spaces 640 between the ribs 650, 660, 670, and 680 at a backside of the fastening portion 600 dampen and attenuate vibrationoccurred during driving the motor, to improve mechanical reliability ofthe stator 6, and contributes to save material of the insulators.

The supporting ribs 650 of the upper insulator 60 a and the lowerinsulator 60 b formed at an inside in contact with the stator core 15along the circumferential direction support an inside surface of thestator core 15.

The reinforcing rib 660 connected between the boss 620 of the fasteninghole 620 a and the supporting rib 650 at each of the fastening portion600 of the upper insulator 60 a and the lower insulator 60 b spreads thefastening force concentrated on the boss 620, and reinforces a strengthof the fastening portion 600.

According to this, the stator 6 can effectively prevent a fasteningportion of the stator 6 suffering from breakage caused by vibration atthe spinning, and shaking and deformation of the rotor 5 even at a largecapacity drum type washing machine having a weight over 1.5 kg only ofthe stator, and a spinning speed ranging 600˜2,000 RPM.

As a positioning projection 630, or 143 b in the vicinity of thefastening hole 620 a of the fastening portion 600 fits in a positioninghole (not shown) in the tub 2, fastening of the stator 6 is easy.

Thus, the positioning projection enables an easy assembly of the stator6 with the tub 2, and a serviceman to make an easy repair at the time ofafter service.

Of course, the positioning projection 630 may be formed on the tub 2,and the positioning hole may be formed in the fastening portion 600.

In the meantime, alike the second embodiment (see FIGS. 17 and 18), theinsulators of the third embodiment are also versatile.

That is, since the separate type upper, and lower insulators of theembodiment are applicable to the core regardless of the total height ofthe core within a certain range of the total height, the separate typeupper, and lower insulators of the embodiment can improve workability onan assembly line.

In the meantime, in a case a metal bushing 800 is press fit to an insideof the fastening hole like the second embodiment (see FIG. 14), asupporting force against the bolt fastening force at the time offastening the stator can be enhanced further.

A fourth embodiment of the present invention will be described withreference to FIGS. 27, and 28. FIG. 27 illustrates a perspective view ofan outer rotor in accordance with a fourth preferred embodiment of thepresent invention, with a partial cut away view, and FIG. 28 illustratesa section of FIG. 27.

Referring to FIGS. 27 and 28, the outer rotor R in accordance with thefourth embodiment of the present invention includes a rotor frame 100 ofa steel plate, to serve as a back yoke too. The rotor frame 100 hassynthetic resin coated on an outside surface thereof.

The rotor frame 100 of a steel plate includes a bottom 110, and asidewall 120 substantially perpendicular to the bottom, wherein thesidewall 120 of the rotor frame 100 also serves as a back yoke, andinside, and outside surfaces of the rotor frame 100 inclusive of anoutside surface of the sidewall 120 have a coat C of synthetic resin.

The synthetic resin is coated by injection molding or the like, to athickness below 3 mm.

The work of the outer rotor of the embodiment is as follows.

Owing to the synthetic resin coated layer C on the surface of the rotorframe 100, even if the outer rotor R is scratched on the surface of therotor during transportation or other situation, the synthetic resincoated layer C is scratched, but the surface of the rotor frame 100inside of the coated layer C is not scratched, to protect the surface ofthe rotor frame 100.

Moreover, because the synthetic resin coated layer C on the surface ofthe outer rotor R serves as a protection film, to prevent water ordetergent from infiltrating into the surface of the rotor frame 100,rusting or corrosion of the surface of the rotor frame 100 can beprevented.

Despite of the anti-rusting heat treatment on a surface of the relatedart steel plate rotor R, though the related art steel plate rotor R hasproblems in that rust occurs on a portion of the surface scratchedduring transportation or other situations as water infiltrates duringuse of the product, or the rust occurs on the surface due to chemicalreaction of the detergent during long time use of the product, the outerrotor R of the present invention can prevent such problems owing to theprotection of the synthetic resin coated layer C on the surface of therotor frame 100.

Particularly, the drawings of the embodiment illustrate that thesynthetic resin is coated on an entire surface of the rotor, it ispreferable that at least the outside circumferential surface of thesidewall 120 of the rotor frame 100 which has a high probability ofcoming into contact with water or detergent, and is liable to bescratched is coated with synthetic resin without fail, even if an entiresurface, such as an inside surface and the outside surface, is notcoated with synthetic resin, taking weight increase of the rotor, and acooling time period increase of the synthetic resin in the injectionmolding caused by the synthetic resin coating into account.

Particularly, because the rotor becomes heavier, and a cooling timeperiod of the synthetic resin becomes longer if a thickness of thecoated synthetic resin is thicker than 3 mm, which is not favorable formass production, it is preferable that the thickness of the syntheticresin is below 3 mm.

In the meantime, the outer rotor R of the embodiment also has a stepportion along a circumferential direction having a supporting surfacefor seating the magnets M on the sidewall 120 of the rotor frame 100.

Moreover, there are a plurality of radial cooling fins 130 around acenter portion of the bottom 110 of the rotor frame 100 of the outerrotor R to blow air toward the stator to cool down heat generated at thestator.

The pass through holes 140 formed in parallel to the cooling fins 130serve as vent holes, the embossed portions 150 between adjacent coolingfins 130 on the bottom 110 of the rotor frame 100 reinforces the rotorframe 100, and water is drained through the drain holes 160 in theembossed portions 150.

A fifth embodiment of the present invention will be described in detail,with reference to FIGS. 29 and 30.

FIG. 29 illustrates a perspective view of an outer rotor in accordancewith a fifth preferred embodiment of the present invention, with apartial cut away view, and FIG. 30 illustrates a section of FIG. 29.

Referring to FIGS. 29 and 30, the outer rotor R includes a rotor framehaving a bottom 110, and a sidewall 120 extended in a substantiallyvertical direction from a circumference of the bottom 110, wherein anannular back yoke Y is mounted on an inside surface of the sidewall 120of the rotor frame 100, and magnets M are in turn attached to an insidesurface of the back yoke Y.

In this instance, it is preferable that a step portion is provided onthe sidewall 120 of the rotor frame of the outer rotor R for forming asupporting surface the magnets M are to be seated thereon.

The annular yoke may be formed as one body without discontinued portion,or one body with discontinued portions.

The annular yoke formed as one body with discontinued portions may abutat the discontinued portions, or the discontinued portions may bewelded, to connect the discontinued portions.

In the meantime, the annular back yoke Y may be a stack of thin plates,or one body without the stacking.

The back yoke Y may be secured to an inside surface of the sidewall 120of the rotor frame of the outer rotor R, or the like.

Moreover, alike the fourth embodiment, it is preferable that the outerrotor of the embodiment also has a plurality of cooling fins 130 arounda center of the bottom 110 of the rotor frame 100, lanced toward an openside of the rotor frame 100.

The pass through holes 140 formed parallel to the cooling fins 130 bylancing serve as vent holes.

There are embossed portions 150 between adjacent cooling fins 130 on thebottom 110 of the rotor frame 100 for reinforcing strength of the rotor,and there are drain holes 160 in the embossed portions 150 for drainingwater.

The work of the embodiment (i.e., the fifth embodiment) is as follows.

As the outer rotor R of the embodiment has the annular back yoke Ymounted on the inside surface of the sidewall 120 of the rotor frame100, and the magnets M attached to the inside surface of the back yoke Yagain, rigidity of the outer rotor R is enhanced owing to the back yokeY.

That is, in a structure in which the back yoke Y is mounted at aposition far from the center of the outer rotor R, and the magnets M aremounted at a position near to the center of the outer rotor R, whichincreases rigidity of the sidewall of the outer rotor R significantlyowing to the back yoke Y which is in close contact with the rotor framesidewall 120 of the outer rotor R, the throbbing of the rotor duringspinning can be prevented, effectively.

Moreover, as the supporting surface at the sidewall 120 of the rotorframe 100 supports the back yoke Y and the magnets M at the same time,mounting of the back yoke is easy.

In the meantime, there are a plurality of radial cooling fins 130 arounda center portion of the bottom 110 of the rotor frame 100 to blow airtoward the stator to cool down heat generated at the stator.

Moreover, the pass through holes 140 formed in parallel to the coolingfins 130 serve as vent holes, and embossed portions 150 between adjacentcooling fins 130 on the bottom 110 of the rotor frame 100 reinforcestrength of the rotor, and drain holes 160 in the embossed portions 150drain water.

Though the present invention has been described taking the drum typewashing machine as an example, the present invention is applicable to apulsator type washing machine, too.

The present invention has the following advantages.

The present invention can reduce material and weight required forfabrication of the stator of the BLDC motor for a drum type washingmachine, simplifies the fabrication process, and enables secure mountingof the stator on a fixing side, such as the tub.

Especially, because the present invention enables to reduce materials ofthe stator core and the insulator, and secure mounting of the stator ona fixing side, such as the tub or the bearing housing like thedivisional core, the present invention is more suitable to a BLDC motorfor a drum type washing machine that has a weight of 1.5 kg only of thestator, and a rotation speed varied 0˜2,000 RPM or over.

The drum type washing machine of the present invention permits easyassembly of the stator and the tub on an assembly line, as well as easyrepair during services.

Moreover, the employment of the helical core HC of which winding is easypermits to prevent waste of base metal, permits easy fabrication, andimproves mechanical reliability and extends a lifetime owing to theenhanced rigidity of the fastening portion of the stator that reducesnoise and vibration.

The coating of synthetic resin on a surface of the outer rotor of steelplate can prevent scratching of the surface of the outer rotor which isliable to occur during transportation.

Along with this, the effective cut off of chemical reaction withdetergent even in a long time use extends a lifetime of the motor andimproves reliability.

The back yoke Y in close contact with an inside surface of the sidewall120 of the rotor frame 100 permits to increase rigidity of the sidewallof the outer rotor R significantly, to prevent the rotor from throbbingoccurred at the time of spinning.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An outer rotor type motor for a drum type washing machine comprising:an annular stator core having multiple layers formed by winding a baseportion in a shape of stripe with Ts projected from the base portion ina helix starting from a bottom layer to a top layer; an insulator insertmolded to cover an outside surface of the stator core for insulating thestator core electrically; fastening portions formed as one body with theinsulator each projected from an inside circumferential surface of thestator core toward a center of the stator, for fixedly securing thestator to the tub, wherein the base portion of the stator core has anarc shaped slots each for reducing stress in winding the core as well asserving as a portion of a fastening hole, and each of the fasteningportion of the insulator formed to cover the slots has a fastening holeformed therein; and an outer rotor of steel plate having a bottom, asidewall extended from the bottom in perpendicular theretosubstantially, and magnets on an inside of the sidewall, wherein therotor frame and a back yoke are formed as one unit so that the rotorframe also serves as the back yoke, and the rotor frame has one coat ofsynthetic resin applied to at least an outside surface of the rotorframe.
 2. The outer rotor type motor as claimed in claim 1, wherein thesynthetic resin is coated by injection molding.
 3. The outer rotor typemotor as claimed in claim 1, wherein the coating of the synthetic resinhas a thickness below 3mm.
 4. An outer rotor type motor for a drum typewashing machine comprising: an annular stator core having multiplelayers formed by winding a base portion in a shape of stripe with Tsprojected from the base portion in a helix starting from a bottom layerto a top layer; an insulator insert molded to cover an outside surfaceof the stator core for insulating the stator core electrically;fastening portions formed as one body with the insulator each projectedfrom an inside circumferential surface of the stator core toward acenter of the stator, for fixedly securing the stator to the tub,wherein the base portion of the stator core has an arc shaped slots eachfor reducing stress in winding the core as well as serving as a portionof a fastening hole, and each of the fastening portion of the insulatorformed to cover the slots has a fastening hole formed therein; and anouter rotor of steel plate having a bottom, a sidewall extended from thebottom in perpendicular thereto substantially, and magnets on an insideof the sidewall, wherein an annular back yoke is mounted on an insidesurface of the sidewall of the outer rotor, and the magnets are attachedto an inside surface of the back yoke.
 5. The outer rotor type motor asclaimed in claim 3, wherein the sidewall of the rotor frame of the outerrotor includes a stepped portion having a supporting surface for seatingthe back yoke and the magnets at the same time.
 6. The outer rotor typemotor as claimed in claim 3, wherein the annular back yoke is one bodywithout discontinued portions in a circumferential direction.
 7. Theouter rotor type motor as claimed in claim 3, wherein the annular backyoke has the discontinued portions in a circumferential direction. 8.The outer rotor type motor as claimed in claim 3, wherein opposite endsurfaces of the discontinued portion of the annular back yoke areconnected by welding.
 9. The outer rotor type motor as claimed in claim3, wherein the annular back yoke includes a stack of thin back yokepieces.
 10. The outer rotor type motor as claimed in claim 3, whereinthe annular back yoke is one whole body.